The National Biomedical ESR Center established in 1976 at The Medical College of Wisconsin is a national resource that produced 174 papers during the previous 5-year funding period. It is the largest biomedical ESR facility in the world with balanced separately funded programs of health related research in the major branches of the field: spin-label probes, organo-metallics and free radicals. Continued support is requested for the next 5 years for improvement of the instrumentation, and for administrative and technical personnel. Throughout the proposal in many ways heavy emphasis is placed on the loop-gap resonator, a development of the Center that is proving to be a major breakthrough in ESR spectroscopy. Core Research consists of 6 instrumental projects entitled #1, A new look at source modulation, #2 Frequency swept ELDOR with a loop-gap resonator, #3 Sensitivity improvement with field effect transistor amplifiers, #4 Development of a microwave bride at K-band, #5 New multifrequency spectrometer station, #6 Microstrip X-band bridge design. One thrust of these projects is to improve the sensitivity of ESR spectroscopy, thereby advancing substantially the range of biomedical problems that can be addressed. Collaborative Research consists of 2 broad programs: A continuation of the present effort averaging about 35 projects per year and resulting in many of the papers, and collaboration with ESR specialists form other institutions in the development of specialized loop gap resonators. Numerous collaborative letters are included. Service will be strengthened by addition of a spectroscopic technician and a synthetic organic chemist. The latter will synthesize spin-labels not commercially available following published procedures. Emphasis is on 15/N substituted labels. An additional capability of the Center open to outside users will be 14/N--15/N ELDOR in which both the labels and the equipment are made available. Training is a continuation of the present very successful program that provides funds for between 5 and 10 young scientists each year to spend 2 weeks at the Center. Dissemination in addition to numerous papers, abstracts and presentations, involves enthusiastic participation in the ESR Newsletter being established at the University of Illinois. The goals os the Center are (a) to be innovative in developing new ESR instrumental modalities, (b) to make available at a national level advanced ESR instruments and expertise, (c) to carry on separately funded health related research in selected areas using ESR spectroscopy.